The invention is directed to a polyurethane adhesive composition that is derived from a glycerol monoester.
Flexible packaging materials are widely used in a variety of areas including food packaging. Flexible packaging is often constructed from film laminates in which a layer of laminating adhesive is disposed between a first film (e.g., a polyethylene terephthalate, polyamide or cellophane film) and a second film. Laminates are often used in food packaging because a desired film property cannot be achieved with a single layer or type of film. By combining films having different properties in the form of a laminate, a desired property or combination of properties can be achieved.
Laminates are typically constructed to have a strong cured bond that is resistant to delaminating stresses resulting from stresses imposed on the laminate when the laminate is formed into other articles such as pouches and bags (e.g., by heat welding) or when the laminate is used. These stresses can include exposure to heat, cold, and humidity and exposure to a variety of compositions that are often stored in bags made from such laminates including, e.g., tomato sauce. Substrates used in food laminates often include metallized films and polyolefin films that exhibit low surface energy. It can be difficult to form strong adhesive bonds to metallized substrates and to substrates with low surface energy.
Polyurethane adhesives have been used in flexible packaging laminates. Polyurethane adhesive compositions are often derived from a two part system in which a first part (e.g., a polyurethane prepolymer) is reacted with a second part (e.g., a polyol). Polyurethane prepolymers are often prepared by blending a liquid polyol with a polyisocyanate.
Glycerol monostearate is a polyol in the form of a white, flaky powder that is solid at room temperature. Glycerol monostearate is reported to have a melting point from 58° C. to 59° C. and is insoluble in water.
There is a need for polyurethane adhesive compositions that are suitable for use in flexible packaging laminates and that exhibit good adhesive properties under a variety of conditions and to a variety of substrates.